Better control of the IO manager thread; improvements to deadlock checking
In the threaded RTS on *nix platforms: - we now start the IO manager thread eagerly at startup time (previously was started on demand). - we now ask the IO manager thread to stop at shutdown - In Timer.c:handle_tick, if it looks like we might be in a deadlock, instead of calling prodOneCapability() which was known to be wrong, we now send a byte down the IO manager's pipe to wake it up. This also avoids a case of double-acquisition of a mutex, which happened if prodOneCapability() was called while the current thread was holding a mutex.
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